1. Field of the Invention
The present invention relates to a solid state image sensing device, particularly to a noise-reduced solid state image sensing device.
2. Description of the Related Art
Sensitivity is the most important performance of the solid state image sensing device. Especially for a multiple-pixel element, it is necessary to increase the density of pixels. When the pixel size decreases, the luminous energy coming into one pixel decreases and thereby, the number of signal charges decreases and noises increase.
For a general solid state image sensing device, many pixels for storing signal charges corresponding to image are arranged horizontally and vertically. Many CCD registers are arranged adjacent to the vertical array of pixels. The vertical CCD register transfers signal charges sent from pixels in the vertical direction. The output end of the vertical CCD register adjoins to a horizontal register CCD and vertically-transferred charges are sent to the horizontal register. The horizontal register transfers signal charges sent from the vertical CCD register in the horizontal direction. A signal charge detecting section connected to the horizontal register detects signal charges and outputs them as a image signals.
The charge detecting section used for the conventional solid state image sensing device is the floating diffusion (hereafter referred to as FD) type, in which signal charges are transferred along the channel of a transfer electrode (one transfer stage normally including a plurality of transfer electrodes) and injected into the FD region through an output electrode. The potential of the FD region is modulated by the charge packet injection, detected by an amplifier, and read as an output signal. During the above period, a reset gate is turned off, and the signal charge packet turns on the reset gate before the next signal charge packet is injected before it is eliminated from a reset drain. When the above operation is repeated synchronously with the clock pulse applied to the transfer electrode, an output waveform corresponding to an image can be obtained. The output signal is proportional to the number of signal charges. For the number of signal charges (more accurately, the number of transferred charges; in this description, it is assumed that no dark charge is present), the number of noise equivalent electrons of the signal charge itself is expressed as the square root of Ns. That is S/N comes to the square root of Ns. When Ns decreases, S/N decreases.
The solid state image sensing device includes, for example, a hi-vision for dynamic images and electronic camera for static images. High resolution is required for the solid state image sensing device for hi-vision. The area of a photo-sensing section is normally specified. To improve the resolution in the specified area, pixel area decreases.
When the luminous energy for unit area is the same, the number of signal charges and S/N decrease. For the electronic camera, noises have not the rms value but the peak value. Therefore, the request for improvement of S/N is larger. For example, even if the noise of amplifier A0 is reduced to 0, the noise determined by the number of signal charges is left. There is large request for reproduction of signals with 10 to 100 signal charges at a high S/N for the hi-vision and electronic camera.
Actually, however, S/N is determined by the noise due to the fluctuation inherent in the number of signal charges as previously mentioned. Therefore, this is a big problem.